216 research outputs found
Precision electroweak tests with scattering
Measurements of the cross section for elastic scattering
with unprecedented precision have recently been proposed. The impact of these
experiments for detecting possible deviations from the standard electroweak
theory is analyzed and compared with that of several other measurements.Comment: 4 pages, 1 figure. To be published in Phys. Rev. D, Brief Reports.
Misprints correcte
Constraining a CP-violating WWV coupling from the W^+W^- threshold cross section at LEP2
The most general form of the and interaction contains a
-violating term which has the same threshold behaviour as the Standard
Model \ee \to \ww cross section. We calculate the cross section as a function
of the corresponding anomalous coupling, and estimate the bounds which can be
obtained from a measurement of the threshold cross section at LEP2. We show how
the effect of the coupling is most pronounced in the angular distributions of
the final-state fermions.Comment: 7 pages, LaTeX file, 5 ps figure
The Height of a Giraffe
A minor modification of the arguments of Press and Lightman leads to an
estimate of the height of the tallest running, breathing organism on a
habitable planet as the Bohr radius multiplied by the three-tenths power of the
ratio of the electrical to gravitational forces between two protons (rather
than the one-quarter power that Press got for the largest animal that would not
break in falling over, after making an assumption of unreasonable brittleness).
My new estimate gives a height of about 3.6 meters rather than Press's original
estimate of about 2.6 cm. It also implies that the number of atoms in the
tallest runner is very roughly of the order of the nine-tenths power of the
ratio of the electrical to gravitational forces between two protons, which is
about 3 x 10^32.Comment: 12 pages, LaTe
New analytic running coupling in spacelike and timelike regions
The new model for the QCD analytic running coupling, proposed recently, is
extended to the timelike region. This running coupling naturally arises under
unification of the analytic approach to QCD and the renormalization group (RG)
formalism. A new method for determining the coefficients of the "analytized" RG
equation is elaborated. It enables one to take into account the higher loop
contributions to the new analytic running coupling (NARC) in a consistent way.
The expression for the new analytic running coupling, independent of the
normalization point, is obtained by invoking the asymptotic freedom condition.
It is shown that the difference between the values of the NARC in respective
spacelike and timelike regions is rather valuable for intermediate energies.
This is essential for the correct extracting of the running coupling from
experimental data. The new analytic running coupling is applied to the
description of the inclusive lepton decay. The consistent estimation of
the parameter is obtained here.Comment: REVTeX 3.1, 12 pages with 3 EPS figures; enlarged version is
published in Phys. Rev.
Radiative Corrections to Fixed Target Moller Scattering Including Hard Bremsstrahlung Effects
We present a calculation of the complete electroweak radiative
corrections to the Moller scattering process e^-e^- -> e^-e^-, including hard
bremsstrahlung contributions. We study the effects of these corrections on both
the total cross section and polarization asymmetry measured in low energy fixed
target experiments. Numerical results are presented for the experimental cuts
relevant for E-158, a fixed target e^-e^- experiment being performed at SLAC;
the effect of hard bremsstrahlung is to shift the measured polarization
asymmetry by approximately +4%. We briefly discuss the remaining theoretical
uncertainty in the prediction for the low energy Moller scattering polarization
asymmetry.Comment: 22 pgs; minor clarifications added and typos fixe
Flavour Mixing, Gauge Invariance and Wave-function Renormalisation
We clarify some aspects of the LSZ formalism and wave function
renormalisation for unstable particles in the presence of electroweak
interactions when mixing and CP violation are considered. We also analyse the
renormalisation of the CKM mixing matrix which is closely related to wave
function renormalisation. We critically review earlier attempts to define a set
of "on-shell" wave function renormalisation constants. With the aid of an
extensive use of the Nielsen identities complemented by explicit calculations
we corroborate that the counter term for the CKM mixing matrix must be
explicitly gauge independent and demonstrate that the commonly used
prescription for the wave function renormalisation constants leads to gauge
parameter dependent amplitudes, even if the CKM counter term is gauge invariant
as required. We show that a proper LSZ-compliant prescription leads to gauge
independent amplitudes. The resulting wave function renormalisation constants
necessarily possess absorptive parts, but we verify that they comply with the
expected requirements concerning CP and CPT. The results obtained using this
prescription are different (even at the level of the modulus squared of the
amplitude) from the ones neglecting the absorptive parts in the case of top
decay. The difference is numerically relevant.Comment: 19 pages, plain latex, one ps figur
Off-Diagonal Hyperfine Interaction and Parity Non-conservation in Cesium
We have performed relativistic many-body calculations of the hyperfine
interaction in the and states of Cs, including the off-diagonal
matrix element. The calculations were used to determine the accuracy of the
semi-empirical formula for the electromagnetic transition amplitude
induced by the hyperfine interaction. We have found that even
though the contribution of the many-body effects into the matrix elements is
very large, the square root formula remains valid to the accuracy of a fraction of .
The result for the M1-amplitude is used in the interpretation of the
parity-violation measurement in the transition in Cs which claims a
possible deviation from the Standard model.Comment: 13 pages, 4 figures, LaTeX, Submitted to Phys. Rev.
Tau Polarimetry with Inclusive Decays
The spin asymmetry parameter characterizing the angular distribution
of the total hadron momentum in the decay of a polarized tau can be calculated
rigorously using perturbative QCD and the operator product expansion.
Perturbative QCD corrections to the free quark result can be
expressed as a power series in and nonperturbative QCD
corrections can be expanded systematically in powers of . The QCD
prediction is . In the decay of a high energy tau into
hadrons, the value of the hadronic energy distribution evaluated
at the maximum hadronic energy fraction can also be calculated
rigorously from QCD.Comment: LateX, 11 pages, no figures, NUHEP-TH-93-
Two-Boson Exchange Physics: A Brief Review
Current status of the two-boson exchange contributions to elastic
electron-proton scattering, both for parity conserving and parity-violating, is
briefly reviewed. How the discrepancy in the extraction of elastic nucleon form
factors between unpolarized Rosenbluth and polarization transfer experiments
can be understood, in large part, by the two-photon exchange corrections is
discussed. We also illustrate how the measurement of the ratio between
positron-proton and electron-proton scattering can be used to differentiate
different models of two-photon exchange. For the parity-violating
electron-proton scattering, the interest is on how the two-boson exchange
(TBE), \gamma Z-exchange in particular, could affect the extraction of the
long-sought strangeness form factors. Various calculations all indicate that
the magnitudes of effect of TBE on the extraction of strangeness form factors
is small, though can be large percentage-wise in certain kinematics.Comment: 6 pages, 5 figures, prepared for Proceedings of the fifth
Asia-Pacific Conference on Few-Body Problems in Physics (APFB2011), Seoul,
Korea, August 22-26, 2011, to appear in Few-Body Systems, November 201
Limits on the Neutrino Mass and Mixing Angle from Pion and Lepton Decays
Motivated by a recent rather surprising conclusion based on the 1992 PDG data
on the pion, kaon and lepton decays that if three generations of neutrinos are
assumed to be massive and mixed, the heaviest neutrino, , could have a
mass in the range, 155~\mbox{MeV} \lsim m_3 \lsim 225~\mbox{MeV}, we have
analyzed the latest 1995 data on the leptonic decays of pion, and
with the assumption that three generations of neutrinos are massive and mixed.
It is shown that when the radiative corrections are included and the constraint
{}from partial decay widths is imposed, the 1995 data are consistent with three
massless neutrinos with no mixing. Various limits on the neutrino mass and
mixing angle implied by the 1995 data are presented together with a critique of
the previous analysis.Comment: REVTeX file, 20 pages and 10 figures (not included). Revision of the
analysis and inclusion of the latest data. The TeX file and the figures
(uuencoded, compressed, tarred file) are available at
http://fermi.pha.jhu.edu/personnel/fornengo/fornengo.htm
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